Thermionic valve circuit



c. c. EAGLEsr-IVELD 2,226,752 THERMI'ONIC' VALVE CIRCUIT Filed May 3, 1939 CHARLES CEC/L EGLFSF/ELD BY gw-M ATTORNEY Patented Dec. 3l, 1940 UNITED STATES PATENT OFFICE THERMIONIO VALVE CIRCUIT tion of Delaware Application May 3, 1939, Serial No. 271,614 In Great Britain May 13, 1938 2 Claims.

This invention relates to thermionic Valve circuits and has tor its object to provide an improved circuit incorporating a Valve which exhibits a saturation eliect within the negative range of at least one of its characteristic curves, i. e. upon plotting in the usual manner the relationship between the current to an electrode (e. g. the anode) and the control-grid volts, the current is represented by a horizontal or substantially horizontal curve over a portion of that range of abscissae which represents negative control-grid potentials. A Valve exhibiting a characteristic of this form is, for example, useful as a so-called amplitude filter in a television receiver for which purpose the ideal shape of characteristic would be constituted by a straight horizontal line extending in the negative direction from the zero ordinate and a sloping straight line extending from the end of the horizontal line to the abscissa.

According to the present invention the desired form of characteristic (if required, one that approaches closely to the said ideal characteristic for an amplitude filter) is obtained by utilising a valve of the type incorporating an auxiliary cathode (i. e. a secondary-emission electrode whose emission is caused to form an anode current greater than that formed by the primary emission from the cathode proper) and by placing a resistance or other appropriate impedance of suitable magnitude in the lead to the anode, in the lead to the auxiliary cathode, or in each of these leads.

In order that the invention may be more readily understood reference will now be made to the accompanying drawing, wherein:

Fig. 1 is a circuit diagram which serves to illustrate the principle of the invention;

Figs. 2 to 4 are graphs showing the results obtained with the circuit of Fig. 1.

The valve I0 shown in Fig. 1 incorporates a cathode I I, a control grid I2, a screening grid I3, an auxiliary cathode I4 and an anode I5. This valve may conveniently have the constructionl shown in Fig. 4 of the article by Jonker and V. Overbeek appearing in pages' 150 to 156 of Vol. 15 of the Wireless Engineer. The heater for the cathode and other accessories such as earthed screens are not shown in the diagrammatic representation of the valve. Fig. 1 also shows a source of bias I6 connected between the cathode and control grid, a high tension battery I1, a resistance I8 included in the lead which connects the anode I5 to the battery, and a resistance I9 in the lead connecting the auxiliary (Cl. Z50-27) cathode I4 to the battery.' The screen-grid I3 is shown connected to the saine point of the battery as that to which resistance I9 is connected but it may have a connection to another point of the battery. f 5

I have plotted characteristic curves of the above-mentioned form of valve utilising resistances I8 and I9 of 10,000 ohms each, an anode voltage of 250 volts and an auxiliary cathode voltage of 150 volts. Figs. 2 'to 4 illustrate the re- 10 sults obtained.

When the resistance I8 is in the position shown but the resistance I9 is omitted the curve of anode current A1 obtained upon variation of the source I6 (Gv) is shown by curve 20 in Fig. 2. 15 The formation of this curve may be explained as follows: When the anode potential is higher than the potential of the auxiliary cathode the secondary emission from the latter will be collected by the anode, whereas if the anode potential is 20 reduced to a value only slightly higher than that of the auxiliary cathode most of the secondary emission will return to the auxiliary cathode. When the resistance I8 is placed in the anode lead a reduction of the negative bias on the con- 25 trol grid results in a reduction of anode potential due to the increase in the anode current and curve 20 accordingly shows a saturation effect at lower values of negative bias.

If on the other hand resistance I9 is placed as 30 shown in Fig. 1 but resistance I8 is omitted we obtain curve-2| of Fig. 3. It will be noted that this curve shows the current AKi to the auxiliary cathode as a function of the control grid bias in contradistinction to Fig. 2' in which anode 85 current was plotted. The formation of curve 2I may be explained as follows. When the negative bias on the control grid is reduced the auxiliary cathode current (-AKi) increases, thus increasing the voltage of the auxiliary cathode un- 40 til it approaches the anode voltage.

If resistances I8 and I9 are used simultaneously we obtain the curve shown in Fig. 4, curve 22 showing the variation of anode current (A1) and curve 23 the variation of auxiliary cathode 45 current (AKD. In this case the screen-grid I3 may advantageously be given a potential of Volts instead of being connected to the end of the resistance I9 as shown in Fig. 1.

It will be noted that in curve 22 the satura- 50 tion portion has a slight upward slope whereas the corresponding curve 23 in Fig. 4 has a slight downward inclination in the saturation region.

It will be clear that by suitably adjusting the value of resistance I 8 the curve may be made hor- 55 izontal. As is obvious from a comparison of Figs. I2 and 4 the effect of inserting resistance I9 is to atten the saturation region of the anode current curve.

Fig. 1 has been used to illustrate the principle of the invention and it is thought that it is unnecessary to show any input or output connections since these may be of any suitable character. The output voltage may be taken from either resistance I8 or resistance I9 alone or, if a push-pull output is desired, from both these simultaneously. In the last mentioned case suitable adjustment of resistances I8 and I9 will produce a true amplitude balance of the voltages.

As is clear from the above explanation and from the accompanying graphs a circuit in accordance with the invention is applicable with great advantage as an amplitude filter in a television circuit.

What I claim is:

1. A circuit comprising an electron discharge tube provided with at least a source of primary electrons, a signal control grid, a source of secondary electrons and an anode, a circuit including a source of negative potential connected between the signal control grid and the primary electron source, means for impressing a positive potential on the anode and on the secondary electron source, and a voltage dropping resistance connected to each of said electrodes having positive potentials impressed thereon, said resistances being of such magnitude that the anode currentgrid voltage and the secondary electron source current-grid voltage tube characteristics exhibit a saturation effect in the negative range thereof.

2. A circuit comprising an electron discharge tube provided with at least a source of primary electrons, a signal control grid, a source of secondary electrons and an anode, a circuit including a source of negative potential connected between the signal control grid and the primary electron source, means for impressing a positive potential on the anode and on the secondary electron source, and a voltage dropping resistance connected to the anode, said resistance being of such magnitude that the anode current-grid voltage tube characteristic exhibits a saturation effect in the negative range thereof. 

